1. Field of the Invention
This invention relates generally to optical transmitters. More particularly, it relates to vertically lasing semiconductor optical amplifiers (VLSOAs) used in combination with a laser source and/or a modulator to create an improved optical transmitter.
2. Description of the Related Art
Today, optical systems are used to transmit information at high speeds over large distances. At a high level, a typical optical system consists of an optical transmitter, an optical receiver and an optical fiber connecting the two. The optical transmitter converts an electrical signal containing data into an optical signal and transmits the signal over the optical fiber. The optical receiver then receives the signal from the optical fiber and converts the signal back to an electrical signal, recovering the original data.
Semiconductor lasers are the most widely utilized light source used in transmitters for optical communications systems. The lasers produce a beam of light which is modulated with the data to be transmitted across the optical system. On-off keying is a common modulation scheme used to impress the data onto the optical beam. On-off keying, as well as other types of modulation, can be applied to the optical beam in two basic ways. The first way is direct modulation of the laser. In this case, the optical beam generated by the source is modulated by varying the current driving the laser source. The second way is external modulation, in which the laser source produces an unmodulated optical beam, which is then modulated by a device external to the laser source (i.e., the external modulator).
One problem with direct modulation, particularly in the case of on-off keying, is that direct modulation can result in frequency chirp. For example, in the case of on-off keying, turning the laser source on and off results in a light pulse that is chirped meaning that the frequency of the light drifts over the length of the pulse. This occurs because the injected carrier density in the laser does not remain constant and thus the frequency of the light pulse output by the laser varies over time. The chirped light pulse has a broader frequency spectrum and therefore is more susceptible to pulse broadening due to dispersion as the light pulse travels across the fiber.
Pulse broadening that results from chirping is especially problematic for optical systems that operate at high speeds and/or over long distances. For this reason, direct modulation currently is not favored for optical systems operating at bit rates greater than 5 Gbps regardless of length, optical systems greater than 160 kilometers (km) in length operating at bit rates of 2 Gbps or higher, and optical systems greater than 320 km in length operating at bit rates of 1 Gbps or higher.
One way to essentially avoid the chirping problem is by using external modulators to impose the data onto the optical beam. By placing an external modulator after the laser source to provide the modulation, the laser source is driven by a constant injection current and therefore produces light at a constant frequency (i.e., without chirp). When the modulator imposes the data onto the light, the output light pulse has very low chirp since the light input into the external modulator has a constant frequency and external modulators generally introduce very little chirp. For this reason, external modulators are used for dispersion sensitive systems (i.e., at higher data rates and longer distances). Examples of external modulators that are often used in optical systems include lithium niobate (LiNbO3) modulators and electro-absorption modulators (EAM).
A common optical transmitter used today is an electro-absorption modulated laser (EML). An EML is an integrated source. Typically, an EML consists of a distributed feedback (DFB) or distributed Bragg reflection (DBR) type laser integrated with an external electro-absorption modulator (EAM). These devices also generally include a conventional semiconductor optical amplifier (SOA) integrated between the EAM and the laser source. The SOA is used to amplify the light output by the DFB or DBR laser prior to being modulated by the EAM.
SOAs are non-lasing optical amplifiers that contain a semiconductor active region. An electrical current is typically used to pump the electronic population in the active region. As an optical signal propagates through the active region it experiences gain due to the stimulated emission. One problem with non-lasing semiconductor optical amplifiers is that the gain depends on the amplitude of the optical signal. This problem is the result of gain saturation, in which there are insufficient carriers in the conduction band to provide the full amount of gain to higher power signals. As a result, a strong optical signal will be amplified less than a weak signal and strong portions of the optical signal will be amplified less than weak portions. This non-linear gain results in distortion of the optical signal and crosstalk between different optical signals propagating simultaneously through the system, significantly limiting the attractiveness of SOAs. The non-linear gain also introduces chirp.
EMLs are currently widely used in medium to long haul fiber-optic systems. Typically, EMLs are used in optical systems operating at bit rates around 1 billion bits per second (Gbps) over about 320 kilometers (km), bit rates around 2.5 Gbps over about 160 to 640 km, or bit rates around 10 Gbps over distances less than 80 km. Below this performance envelope, a laser source that is directly modulated can be used in place of an EML because such applications are not as susceptible to dispersion and chirp. Above this envelope however, an external lithium niobate (LiNO3 modulator) typically is used to reduce dispersion and chirp, which is a limiting factor in optical systems operating above these ranges.
EML sources currently have two major limitations which limit their performance. First, the maximum output power of the EML is limited to approximately 1 mW. The DFB and DBR lasers have limited output power. This power can be boosted to approximately 10 mW by integrating an SOA between the laser source and the EAM. However, the EAM has approximately 10 dB of loss associated with it. Therefore, the output power of the EML+SOA+EAM combination is limited to approximately 1 mW. This output cannot be amplified further by a second SOA integrated after the EAM because the second SOA would introduce TDM crosstalk (intersymbol interference) in the modulated optical signal. Thus, EMLs are currently limited to a maximum output power of approximately 1 mW.
The second limitation of the EML is that it is limited to a single laser source integrated on a chip. To produce a wavelength division multiplexed (WDM) optical signal using EML sources, a separate EML would be used to generate each wavelength channel. Then, the individual channels would be combined using a waveguide coupler or wavelength division multiplexer into a single WDM optical signal. However, waveguide couplers and wavelength division multiplexers have high losses associated with them. Since the output power of the EMLs is already limited, the loss introduced by these devices makes this combination impractical. In addition, an SOA cannot be integrated after the waveguide coupler or wavelength division multiplexer due to the TDM and WDM crosstalk that the SOA would introduce into the optical signal.
What is needed is a small and inexpensive optical transmitter that can produce high speed, high power optical signals that can be used in single wavelength, time division multiplexed and wavelength division multiplexed optical signals.